Added #ifdef _DISABLE_SKIP in MBCoding(). If this is defined, there
will be __no skipped macroblocks__ in P-frames!
This is of course bad (bigger files), but it's needed for clean B-frame
encoding until we have a workaround for the SKIP-flaw in the standard. :-(

ProfDrMorph's new (unoptimized) Luma Masking code:
luminance masking now works with relative values instead of absolute ones
the new code raises/lowers oberall quant for especially bright/dark frames ( Weber-Fechner law: humans see less differences in bright scenes )
every pixel with a very low lumi value is set to 0 for better compressability ( again the Weber-Fechner law; below a certain intesity humans won't see anything )

- Added quiet_encore symbol so transcode can use XviD with its divx4 API
- Added comments
- Added possible ToDos and pointed out 2 major problems :
+ Reentrance
+ the quality setting for the encoder part